Corning Incorporated, Corning, N.Y., has marketed glass lenses which polarize radiation in the red to near infrared portion of the radiation spectrum for a number of years under the trademark POLARCOR.RTM.. Those lenses are prepared in accordance with the method described in U.S. Pat. No. 4,479,819 (Borrelli et al.). As is explained in that patent, the method involves four fundamental steps:
(a) a glass article of a desired configuration is fashioned from a composition containing silver and at least one halide from the group of chloride, bromide, and iodide;
(b) that glass article is heated to a temperature above the strain point, but not higher than about 50.degree. C. above the softening point of the glass, for a sufficient period of time to generate crystals therein of AgCl, AgBr, and/or AgI;
(c) that crystal-containing article is elongated under stress at a temperature above the annealing point, but below that at which the glass exhibits a viscosity of about 10.sup.8 poises, such that the crystals are elongated to an aspect ratio of at least 5:1, and thereafter;
(d) that article is exposed to a reducing environment at a temperature above about 250.degree. C., but no higher than about 25.degree. C. above the annealing point of the glass, for a sufficient length of time to develop a chemically reduced surface layer on the article wherein at least a portion of the elongated silver halide particles is reduced to elemental silver.
Three different general base types of silver halide-containing glass compositions are disclosed therein: (1) glasses containing copper which exhibit photochromic behavior; (2) glasses having similar base compositions, but wherein copper is absent so that the glasses do not exhibit photochromic behavior; and (3) glasses having compositions in the alkali metal oxide borosilicate system containing high levels of B.sub.2 O.sub.3 which do not display photochromic behavior. Readily-reducible metal oxides, such as PbO and Bi.sub.2 O.sub.3 will be avoided.
In the preferred embodiment of the first two types of glasses, each had a base composition consisting essentially, expressed in terms of weight percent on the oxide basis, of 6-20% R.sub.2 O, wherein R.sub.2 O consists of 0-2.5% Li.sub.2 O, 0-9% Na.sub.2 O, 0-17% K.sub.2 O, and 0-6% Cs.sub.2 O, 14-23% B.sub.2 O.sub.3, 5-25% Al.sub.2 O.sub.3, 0-25% P.sub.2 O.sub.5, 20-65% SiO.sub.2, 0.004-0.02% CuO, 0.15-0.3% Ag, 0.1-0.25% Cl, and 0.1-0.2% Br, the molar ratio R.sub.2 O:B.sub.2 O.sub.3 ranging between about 0.55-0.85, where the composition is essentially free from divalent metal oxides other than CuO, and the weight ratio Ag:(Cl+Br) ranging about 0.65-0.95. In the second type of glass, CuO is omitted from the composition such that the glass does not demonstrate photochromism.
The third type of glass containing silver halide crystals may contain copper and consists essentially, expressed in terms of weight percent on the oxide basis, of 5-12 alkali metal oxides, 1-15% Al.sub.2 O.sub.3, 27-35% B.sub.2 O.sub.3, and the remainder SiO.sub.2, the molar ratio (R.sub.2 O--Al.sub.2 O.sub.3):B.sub.2 O.sub.3 preferably being less than 0.25.
The elongated metallic silver particles can polarize light in the visible and/or near infrared regions of the radiation spectrum depending upon the size and aspect ratio of the elongated silver halide crystals. Nevertheless, inasmuch as the preferred reducing treatment penetrates the glass only to a depth of about 20 microns, a large population of photochromic silver halide crystals will be left in the glass matrix of the first type of glass described above. (The depth to which the reducing heat treatment penetrates is controlled by the time and temperature of the treatment.) The phenomenon of photochromic behavior is of no consequence when the polarizing effect takes place in the infrared regime of the radiation spectrum. Photochromism becomes a problem, however, where polarization in the visible portion of the radiation spectrum is desired.
As was discussed above, copper-free, silver halide-containing glasses which display no photochromic behavior have been prepared. Unfortunately, whereas photochromism was not displayed by the glasses, the silver was reduced to the metallic state during melting of the glass batch or during the subsequent heat treatment to generate silver halide crystals. That action is evidenced in the formation of a red colored glass. This premature reduction of the silver precludes the formation of silver halide crystals in the subsequent heat treatment. As indicated in U.S. Pat. No. 4,479,819, supra, the elongated silver halide crystals, wherein at least a portion thereof is reduced to elemental silver, imparts the polarizing capability to the glass.
The mechanism via which the inclusion of copper inhibits the reduction of Ag.sup.+ ions to Ag.degree. metal has been explained as follows. The presence of Cu.sup.+2 ions in the glass composition provides protection against the reduction of Ag.sup.+ ions to Ag.degree. by the reaction Cu.sup.+2 .fwdarw.Cu.sup.+. Hence, the elimination of copper from the glass removes this protection, thereby allowing the reaction Ag.sup.+ .fwdarw.Ag.degree. to take place.
Therefore, the primary objective of the present invention was to develop glass compositions containing silver halides which, when heat treated to generate silver halide crystals that can be elongated to impart polarizing properties to the glasses, would not exhibit photochromism when exposed to radiation in the ultraviolet/visible portions of the radiation spectrum, and wherein silver ions present in the glasses would not be reduced to metallic silver during the melting process or the heat treatment step.